“Include identifying and/or sequencing information in the message, and keep retrying until confirmation is received.”
While discussing the Saga pattern, we made the implicit assumption that communication between the saga and the affected components is reliable. We pictured a group of people standing in the same office and discussing the process without external disturbances. This is a useful way to begin, because it allows you to focus on the essential parts of a conversation; but we know that life does not always work like that—in particular in distributed systems, where messages can be lost or delayed due to unforeseeable, inexorable subsystem failures.
Fortunately, we can treat the two concerns on separate levels: imagining that the conversation occurs in a busy, noisy office does not invalidate the basic structure of the business process we are modeling. All that is needed is to deliver every message more carefully, transmitting it again if it is overshadowed by some other event. This second level is what the Business Handshake pattern is all about.